Method for manufacturing a plastic encapsulated semiconductor device and a lead frame therefor

ABSTRACT

A method for manufacturing a plastic encapsulated semiconductor device is provided which comprises the steps of: clamping by upper and lower molds at least external leads and strips of a semiconductor device assembly formed using a lead frame which has a first connecting band connected to the external leads extending from one side of a substrate support further used as a heat sink, and a second connecting band connected to the strips having portions of small cross-sectional areas and of a predetermined length and extending from the other side of the substrate support, the cross sections being perpendicular to an extending direction of the strips, so that the substrate support may float in a cavity formed by the upper and lower molds and parts of the portions of small cross-sectional areas may be disposed in the cavity and the remaining parts thereof may be disposed between the upper and lower molds; injecting a plastic into the cavity; and cutting the portions of small cross-sectional areas of the strips which extend outside a plastic encapsulating housing along the end face of the plastic encapsulating housing and cutting a connecting portion between the external leads and the first connecting band. 
     Also provided is a lead frame having the strips as described above. 
     A plastic layer of desired thickness can be formed on the rear surface of the substrate support. Cutting of the strips from the plastic encapsulating housing is easy and reliable.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to a method for manufacturing a plasticencapsulated semiconductor device which can be used with relativelylarge power and a lead frame therefor.

II. Description of the Prior Art

Plastic encapsulated semiconductor devices are superior to metalencapsulated semiconductor devices in ease in mass production andmanufacturing costs. However, the plastic encapsulated semiconductordevices are inferior to the metal encapsulated semiconductor devices inradiation of heat when they are operated. Plastic encapsulation ofsemiconductor devices has recently been developed. A high powertransistor manufactured by plastic encapsulation has been proposed. Inthis case, sufficient consideration is taken to allow the radiation ofheat.

In a transistor adhered on a metal substrate support and encapsulated byplastic, for example, the lower surface of the substrate support is notcovered with plastic but exposed. The substrate support is mounted on aradiator to radiate heat. However, in this case, the substrate supportmust be electrically insulated from the radiator. The packagingoperation of the semiconductor device on the radiator through aninsulating plate is complicated and cumbersome.

On the other hand, a plastic encapsulated power transistor is proposedwherein a thin plastic layer is formed on the lower surface of thesubstrate support during plastic encapsulation and an insulating plateis not required for mounting the power transistor on the radiator.However, in this case, at the time of plastic encapsulation, only theside of the lead frame from which extend the external lead is clamped bythe upper and lower molds with a transistor assembly which has theexternal lead on one side. Plastic is injected while the substratesupport is floating in a cavity defined by the molds. Thus, thesubstrate support may be bent in the cavity due to the injectionpressure of the plastic. As a result, it is very difficult toencapsulate in plastic while keeping the substrate support in a properposition, thus, resulting in non-uniformity in the thickness of theplastic layer on the lower surface of the substrate support anddegrading radiation characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing a plastic encapsulated semiconductor device and a leadframe therefor wherein, in manufacturing a plastic encapsulatedsemiconductor device of a structure which has a thin plastic layer onone surface of a substrate support which supports a semiconductorsubstrate and also serves as a heat sink, to the other surface of whichthe semiconductor substrate is adhered, the thickness of the thinplastic layer is uniformly formed and with high precision.

In order to achieve the above object of the present invention, there isprovided a method for manufacturing a plastic encapsulated semiconductordevice and a lead frame therefor wherein a semiconductor device assemblyis formed, using a lead frame an external lead of which extends to oneside of a substrate support and strips of which extend to the other endof the substrate support, said strips each having a portion ofcross-sectional area are made small for a predetermined length, thecross sections being perpendicular to the extending direction of saidstrips; the external lead and strips are clamped by upper and lowermolds so as to float the substrate support in a cavity of the molds andto position part of the portion of each of the strips with smallcross-sectional area in the cavity of the molds and the remaining partof the portion of each of the strips between the molds for plasticencapsulation; and a connecting portion between a first connecting bandand the external lead and the portion of each of the strips with smallcross-sectional area extending from a plastic encapsulating housing tothe outside are cut.

The substrate support is, therefore, supported properly at the time ofplastic encapsulation. The substrate support may not be bent due toinjection pressure of the plastic, so that a plastic layer is uniformlyformed on the rear surface of the substrate support with high precision.Further, the portions of the strips with small cross-sectional areas areeasily cut without damaging the encapsulating housing.

The above and other objects and features of the present invention willbecome apparent from the following detailed description of the preferredembodiments when taken in conjunction with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are sectional views showing the structures of conventionalplastic encapsulated power transistors, respectively;

FIG. 3 is a plan view of a conventional lead frame;

FIG. 4 is a view illustrating the state of plastic encapsulation forforming the plastic encapsulated power transistor of FIG. 2 using thelead frame of FIG. 3;

FIGS. 5A and 5B are a plan view and a sectional view, respectively, of alead frame according to one embodiment of the present invention;

FIGS. 6, 7 and 8 are views illustrating the plastic encapsulationprocess to the completion of manufacture according to a method formanufacturing the plastic encapsulated semiconductor device of thepresent invention;

FIGS. 9 and 10 are views showing examples of portions of the strips withsmall cross-sectional areas, respectively;

FIGS. 11A and 11B are a plan view and a sectional view, respectively, ofthe main part of a lead frame according to another embodiment of thepresent invention; and

FIGS. 12, 13 and 14 are plan views of the main part of a lead frameaccording to still another embodiment of the present invention,respectively.

DETAILED DESCRIPTION OF THE PRIOR ART

FIG. 1 is a sectional view of a conventional power transistor of theplastic encapsulated structure. The lower surface of a substrate support2 on which a transistor element 1 is adhered and which serves as a heatsink is not covered with a plastic encapsulating housing 3 but exposed.A through hole 4 is formed for mounting the power transistor on theradiator with a screw. Reference numeral 5 denotes a protective plasticportion and reference numeral 6 denotes an external lead. When theplastic encapsulated power transistor with the above structure is to bemounted on the radiator (not shown), the exposed lower surface of thesubstrate support 2 must be thermally coupled with the radiator but mustbe electrically insulated therefrom. This electrical insulation may beperformed by insertion of an insulating plate such as a mica plate.

With the above structure, the heat radiation effect is guaranteed.However, the insulating plate must be inserted between the radiator andthe substrate support when the substrate support is to be mounted on theradiator, resulting in a complicated packaging operation. Furthermore,the insulating plate must be properly inserted between the radiator andthe substrate support. When the insulating plate, the radiator and thesubstrate support are to be integrally adhered, they may be misaligned.Thus, electrical insulation cannot be guaranteed. Therefore, as shown inFIG. 2, a plastic encapsulated power transistor is proposed wherein athin plastic layer 7 is formed on the lower surface of the substratesupport 2 and the insulating plate is not required.

FIG. 3 is a plan view of a lead frame which is conventionally used forpackaging the plastic encapsulated power transistor of FIGS. 1 and 2.External leads 6, 10 and 11 of the power transistor extend in onedirection from a connecting band 9 on which apertures 8 for determiningthe feed pitch and positioning the substrate support 2 at the time ofplastic encapsulation are formed. As shown in FIG. 4, the substratesupport 2 is connected to the end of the external lead 6. As shown inthe leftmost transistor, the transistor is packaged in such a mannerthat the transistor element 1 is adhered, metal wires 12 are connectedbetween the external leads 10 and 11 and electrodes of the transistorelement 1 corresponding thereto, and a protective plastic portion 5 isformed.

A transistor assembly is obtained, using the lead frame as describedabove. This transistor assembly is formed into a plastic encapsulatedstructure shown in FIG. 2 in the following manner. As shown in FIG. 4,the substrate support 2 of the transistor assembly isencapsulatedsemiconductor device of FIG. 2 is manufactured.

As is apparent from FIG. 4, when the plastic encapsulated structure ofFIG. 2 is to be obtained by using the lead frame of FIG. 3, plastic isinjected into the cavity while only the side on which the external leadsare formed is clamped between the upper and lower molds. The substratesupport 2 may be bent within the cavity due to the injection pressure ofthe plastic. Therefore, it is very difficult to dispose the substratesupport 2 in a proper position. If the substrate support 2 is bent, theuniform thickness of the thin plastic layer 7 is not obtained. Further,this non-uniformity in thickness directly results in degradation ofradiation characteristics.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 5A and 5B are views illustrating the structure of a lead frameaccording to the present invention in which FIG. 5A is a plan viewthereof and FIG. 5B is a sectional view thereof along the line U--U.

Two strips 15 and 16 extend from a side of the substrate support 2 whichis opposite to the side to which the external lead 6 is connected. Thestrips 15 and 16 are connected to a second connecting band 17. Portions18 and 19 of small cross-sectional areas are formed at the strips 15 and16, respectively. Apertures 20 formed in the second connecting band 17fit with parts of a mold for alignment in the plastic encapsulationprocess. As shown in FIG. 5B, the thickness of the strips 15 and 16 issmaller than that of the substrate support 2. A predetermined step isformed between the rear surfaces of the strips 15 and 16 and the rearsurface of the substrate support 2. Thus, the portions 18 and 19 arethinner than any other portions.

FIG. 6 is a view illustrating the state of plastic encapsulation of thetransistor assembly formed by using the lead frame according to thepresent invention. The plastic 30 is injected into the cavity formedbetween the upper and lower molds 13 and 14 in the same manner as in theconventional plastic encapsulation. However, when the lead frameaccording to the present invention is used, as shown in the figure, theexternal lead 6 of the lead frame is clamped by the upper and lowermolds 13 and 14 on one side. At the same time, the strips 15 and 16 andthe second connecting band 17 are clamped by the upper and lower molds13 and 14 on the other side. Projections (not shown) of the upper mold13 fit in the apertures 8 formed in the first connecting band 9.Simultaneously, projections 21 of the upper mold 13 fit in the apertures20 of the second connecting band 17. Reference numeral 22 denotes aprojection of the upper mold 13 which forms a through hole for mountingthe semiconductor device to a radiator with a screw.

According to the present invention, the substrate support 2 of the leadframe is supported by the external lead 6 and the strips 15 and 16 whichare clamped by the upper and lower molds 13 and 14, and thus floats inthe cavity of the molds. The first and second connecting bands 9 and 17are clamped by the upper and lower molds 13 and 14, as described above.Further, since the projections of the upper mold 13 are fitted in theapertures formed in the first and second connecting bands 9 and 17, thefirst and second bands 9 and 17 are not allowed to move horizontally.Thus, the floating condition of the substrate support 2 is properlycontrolled.

As is apparent from FIG. 6, the cavity formed by the upper and lowermolds 13 and 14 terminates at a small cross-sectional area portion 18(19) of the strip 15 (16). Therefore, the small cross-sectional areaportion 18 (19) of the strips 15 (16) extends from the encapsulatinghousing to the outside after plastic encapsulation is completed. Inaccordance with the shape of the extending portion of the strips,effects to be described later will be obtained in the cutting operation.

FIG. 7 is a perspective view illustrating the condition after theplastic encapsulation is completed. As shown in the figure, the plasticencapsulating housing has a thin portion 23 with the through hole 4 fora screw disposed therein and a thick portion 24. A step is formedbetween, the thin portion 23 and the thick portion 24. The head of thescrew mounted in the through hole 4 does not extend higher than the topof the thick portion 24.

The external leads 6, 10 and 11 are cut from the first connecting band 9along the line X--X and the strips 15 and 16 are cut at the portions 18and 19 of small cross-sectional areas along the line Y--Y so as toproduce a plastic encapsulated transistor shown in FIG. 8. Since theportions 18 and 19 extending from the thin portion 23 of the plasticencapsulating housing have cross-sectional areas smaller than those ofother portions, the strips 15 and 16 are easily cut along the line Y--Ywhen bent in the vertical direction indicated by an arrow Z--Z. Sincethe portions 18 and 19 are partially exposed at the end face of the thinportion 23 of the plastic encapsulating housing, the thickness of thestrips 15 and 16 is thinner at the end face of the plastic encapsulatinghousing. Further, since a step is formed between the portion 18 and thestrip 15 and between the portion 19 and the strip 16, the substratesupport 2 is brought into contact with the plastic material at arelatively long length from the end face of the plastic encapsulatinghousing to the transistor element 1. Therefore, water or the like maynot enter through the cut surface. The strips 15 and 16 may be cut by apress machine or the like. However, in order to substantially align thecut surface of the strip with the end face of the plastic encapsulatinghousing with high precision, it is preferable to bend and cut the stripsby brittle fracture under bending stress because cutting by the pressmachine may damage the encapsulating housing, resulting in poorappearance.

In the transistor manufactured according to the method of the presentinvention, the cut surfaces of the strips 15 and 16 are exposed at theend face of the plastic encapsulating housing. However, since the stepis formed between the rear surface of the substrate support 2 and therear surface of the strips 15 and 16, as shown in FIG. 5, a sufficientspace is formed between the cut surfaces and the rear surface of theplastic encapsulating housing which is mounted to the radiator.Short-circuiting does not occur at the cut surfaces of the strips.

In the above embodiment, parts of the strips 15 and 16 are made thin inorder to reduce the cross-sectional areas of the strips at these parts.However, as shown in FIG. 9, the thickness of the strips may be keptconstant but parts thereof may be narrowed for this purpose.Alternatively, as shown in FIG. 10, holes 25 and 26 may be formed in thestrips 15 and 16, respectively, to substantially partially reduce thewidth of the strips 15 and 16. The plastic for plastic encapsulationused according to the method of the present invention preferably hashigh thermal conductivity. The thickness of the plastic layerimmediately under the substrate support is preferably 0.3 to 0.5 mm inconsideration of heat radiation and electrical insulation. With thethickness within this range, better results are obtained.

In the plastic encapsulated semiconductor device with an insulationstructure formed by the process described above, as is also apparentfrom FIG. 6, a thin layer of plastic 30 is formed on the rear surface ofthe substrate support, to the upper surface of which the transistorelement 1 is adhered. The layer of plastic 30 has thermal conductivitylower than that of the substrate support 2 comprising a metal plate.Therefore, the plastic layer on the rear surface of the substratesupport 2 must be as thin as possible and must comprise a plastic ofvery high thermal conductivity. In consideration of this, although avery thin plastic layer is formed on the rear surface of the substratesupport 2, the plastic film may peel off the substrate support 2 due toheat shrinkage therebetween, thus resulting in changes of heatdissipation characteristic over time, degradation of resistance tohumidity, degradation of mechanical strength, and degradation ofelectrical insulation.

The present invention provides a plastic encapsulated semiconductordevice with an insulation structure which eliminates the above problem.The characteristic feature of the present invention resides in thatgrooves or bores are formed in at least part of a surface opposing thesemiconductor substrate adhered surface of the substrate supportconnected to one of the external leads extending in the same directionfrom the connecting band.

The above feature will be described in detail with reference to theaccompanying drawings. FIG. 11A is a plan view of the main part of alead frame for a plastic encapsulated semiconductor device according toanother embodiment of the present invention, and FIG. 11B is a sectionalview thereof along the line W-W of FIG. 11A. A plurality of grooves 27are formed perpendicularly to the direction indicated by an arrow T. Thewidth, depth and pitch of these grooves are not limited but may bechanged as needed. When a thin plastic layer is formed on athree-dimensional surface, the surface area of the substrate supportwhich is brought into contact with the plastic layer is increased.Therefore, heat dissipation characteristics are, of course, improved andthe plastic layer is strongly adhered to the substrate support 2. Thus,the plastic layer does not peel off the substrate support 2, althoughthe layer tends to peel off the flat surface of the substrate support 2.

An increase in the surface area is proportional to the total area ofinner walls of the grooves. Assume that the number of grooves is definedas n, the length of each groove is defined as l, and the depth of thegrooves is defined as d. An increase ΔS of the surface area is given bythe relation: ΔS=2nld. For example, assume that fifteen parallel groovesof 200 μm width and 100 μm depth are formed in a substrate support of 15mm length and 10 mm width. The surface area is calculated as150+(2×15×10×0.1)=180 mm², whereas the surface area of the substratesupport without grooves is calculated as 150 mm². Thus, an increase ΔSof 20% is obtained.

FIGS. 12, 13 and 14 show other examples of three-dimensionally patternedsurface of the substrate support 2, respectively. A plurality of grooves27 are formed in a lattice shape on the rear surface of the substratesupport 2 in FIG. 12; a plurality of grooves 27 parallel to thedirection indicated by the arrow T are formed thereon in FIG. 13; and aplurality of bores each of which has predetermined diameter and depthare formed thereon in FIG. 14. The same effects as described above areobtained in all modifications of the grooves 27. Further, the plastic isinjected in the molds in the direction indicated by the arrow T. If thethree-dimensional patterning process as shown in FIG. 13 is performed,the flow of the plastic in the molds is very smooth, so voids or thelike are easily eliminated, resulting in good appearance and theimprovement of electrical insulation of the device.

As is apparent from the above description, a plastic encapsulatedsemiconductor device which has a thin plastic layer immediately underthe substrate support which also serves as the heat sink is manufacturedwith high precision according to the present invention.

What is claimed is:
 1. A method for manufacturing a plastic encapsulatedsemiconductor device, comprising the steps of:(a) assembling asemiconductor substrate with a lead frame having a first connecting bandconnected to external leads of a semiconductor device assembly extendingfrom one side of a substrate support serving as a heat sink, and asecond connecting band connected to a plurality of strips of saidsemiconductor device assembly extending from another side of saidsubstrate support and having portions of small cross-sectional areas andof predetermined lengths, the small cross-sectional areas beingperpendicular to an extending direction of said plurality of strips, andsaid plurality of strips being thinner than said substrate support andrear surfaces of said plurality of strips being located above a rearsurface of said substrate support; (b) clamping said external leads andsaid plurality of strips between upper and lower molds such that partsof said portions of small cross-sectional areas of said plurality ofstrips are disposed in a cavity formed by said upper and lower molds andthe remainder of said portions of small cross-sectional areas aredisposed between said upper and lower molds separate from said cavityand that said substrate support floats in said cavity with the rearsurface of said substrate support being spaced apart from a bottomsurface defining said cavity by a distance in the range from 0.3 to 0.5mm; (c) injecting an encapsulating plastic into said cavity; and (d)cutting a connecting portion between said external leads and said firstconnecting band, and said portions of small cross-sectional areas ofsaid plurality of strips, extending externally from an encapsulatingplastic housing formed by said injecting step along a surface of theencapsulating plastic.
 2. A method according to claim 1, wherein saidcutting step (d) includes the step of applying a bending stress to saidsecond connecting band to which said plurality of strips are connectedso as to cut said portions of small cross-sectional areas by brittlefracture.